Diamond has a high hardness, good abrasion resistance, a low compressibility and a low coefficient of thermal expansion. Diamond can also have a very high coefficient of thermal conductivity and it can be an extremely good electrical insulator. This makes diamond a desirable material for many applications. For example, by making use of its thermal conductivity, diamond can be an excellent heat spreader material for electronic devices.
In certain electronic devices, the ability to dope diamond with nitrogen is important in order to pin the Fermi Level.
Synthetic diamond material synthesized using high pressure-high temperature (HPHT) synthesis techniques typically contains significant concentrations of nitrogen impurities, particularly single substitutional nitrogen (Ns0), making it yellow. To avoid this, specific precautions may be taken to exclude nitrogen from the synthesis environment. In addition, diamond material produced using HPHT synthesis techniques exhibits strongly differential uptake of nitrogen impurity on the surfaces with different crystallographic orientation (which are the surfaces corresponding to differing growth sectors) that form during synthesis. The diamond material therefore tends to show zones with different colours, resulting from the differing nitrogen impurity concentrations in different growth sectors. In addition, it is hard to control the HPHT synthesis process sufficiently to give a uniform and desired nitrogen concentration throughout even a single growth sector within the synthesized diamond material. Furthermore, in HPHT synthesis, it is typical to see impurities resulting from the synthesis process and the catalysts used—examples would be inclusions comprising cobalt or nickel—features that can result in localised and inhomogeneous strain that degrade the mechanical, optical and thermal properties.
Using a CVD method (such as the process described in U.S. Pat. No. 7,172,655) it is possible to synthesize diamond material that contains significant concentrations (up to approximately 10 ppm [parts per million]) of Ns0, but such diamond is normally brown in colour. This brown colour is believed to be due to the presence of defects other than Ns0 incorporated in the material, thought to be caused by the addition of nitrogen to the CVD synthesis environment.
Intrinsic diamond material has an indirect bandgap of about 5.5 eV and is transparent in the visible part of the spectrum. Introducing defects, also referred to as “colour centres”, which have associated energy levels within the band gap, gives the diamond a characteristic colour which is dependent on the type and concentration of the colour centres. This colour can result from absorption or photoluminescence, or a combination of the two, but generally absorption is the dominant factor. For example, it is well known that the Ns0 defect causes absorption at the blue end of the visible spectrum which, by itself, causes the material to have a yellow colour. Similarly, it is known from Walker (J. Walker, ‘Optical Absorption and Luminescence in Diamond’, Rep. Prog. Phys., 42 (1979), 1605-1659) that when such yellow material is irradiated with high energy electrons to create vacancies (sites in the crystal lattice from which carbon atoms have been displaced), and annealed to cause the migration and eventual trapping of vacancies at nitrogen impurity atoms, NV centres are formed.
EP 0 671 482, U.S. Pat. No. 5,672,395 and U.S. Pat. No. 5,451,430 describe methods of reducing undesired defect centres in CVD diamond using a HPHT treatment, and U.S. Pat. No. 7,172,655 applies an annealing technique to reduce the brownness of a single crystal stone. The most complete removal of brown colour is achieved at annealing temperatures above about 1600° C. and generally requires diamond-stabilising pressures. However, such treatment is an expensive and complicated process in which yields can be seriously affected by cracking of stones etc. Furthermore, due to defect diffusion, such an annealing strategy is not necessarily consistent with avoiding nitrogen aggregation or with the fabrication of high performance electronic devices, where controlling the location of defects may be important. It is therefore considered desirable to be able to directly synthesize diamond material that is not brown but retains the desired high concentration of Ns0 using CVD methods.
There are many variations of the CVD method for the deposition of diamond which are now well established and have been described extensively in the patent and other literature. The method generally involves providing a source gas which, on dissociation to form a plasma, can provide reactive gas species such as radicals and other reactive species. Dissociation of the source gas is brought about by an energy source such as microwaves, RF energy, a flame, a hot filament or a jet based technique, and produces reactive gas species which are allowed to deposit onto a substrate and form diamond. Most of the recent art is focused on the use of hydrogen-based (H-based) plasmas, typically comprising H2 with small additions of methane, typically in the range 1 to 10 volume % (see, for example, J. Achard et al, “High quality MPACVD diamond single crystal growth: high microwave power density regime”, J. Phys. D: Appl. Phys., 40 (2007), 6175-6188), and oxygen or oxygen-containing species typically at the level of 0 to 3 volume % (for example Chia-Fu Chen and Tsao-Ming Hong, Surf. Coat. Technol., 54/55 (1992), 368-373). Hereinafter, oxygen and oxygen-containing species will be referred to collectively as “O-containing species” and are formed from the O-containing sources in the source gas.
Deliberate nitrogen additions into the synthesis environment are known (e.g. Samlenski et al, Diamond and Related Materials, 5 (1996), 947-951), typically with the purpose of enhancing the growth rate, or improving the quality of the diamond in some other way e.g. reducing stress and cracking (WO2004/046427). In these processes, whilst the addition of nitrogen into the synthesis environment does introduce some level of nitrogen into the solid, this is not the primary intent and the overall method is generally to minimise the inclusion of the nitrogen and the other associated defects in the diamond material that is synthesised. One exception is where the intent is specifically to create colour centres in the diamond material in the form of nitrogen defects (e.g. WO2004/046427), however such diamond material is of little use in the applications envisaged here, because of the high defects concentrations other than single substitutional nitrogen incorporated into the diamond material.
WO2004/063430 discloses high pressure and microwave power density (MWPD) to be important for the growth of synthetic CVD diamond material with a low defect concentration (i.e. what is generally termed ‘high quality’ synthetic CVD diamond material).
Whilst most of the recent art is focused on H-based plasmas comprising little or no O-containing species, there are also references to the importance of O-containing species in the etching of non-diamond carbon, in particular in the context of the synthesis of polycrystalline diamond by CVD methods (see, for example, Chen et al., Phys. Rev. B, vol 62 (2000), pages 7581-7586; Yoon-Kee Kim et al., J. Mater. Sci.: Materials in Electronics, vol 6 (1995), pages 28-33), and in synthesizing “high colour” diamond which is free from the impurities which arise when using H-based plasmas in synthesis processes runs at similar pressures and powers. Critically, in this area of prior art, nitrogen incorporated into the diamond material is considered to be one of the defects which is being minimised, and the methods taught reduce the nitrogen content along with other defect types (for example WO 2006/127611).
On the basis of the above, there remains a need for a CVD process for producing single crystal diamond in which the defect content can be controlled and the synthetic CVD diamond product thereof. There is also a need for high colour (i.e. high quality) synthetic CVD diamond material per se.
In particular, there is a need for a CVD diamond material, obtained by direct synthesis, with a relatively high nitrogen content that is uniformly distributed, and which is free of other defects, such as inclusions, normally associated with HPHT synthesis processes. Furthermore, there is a need for such CVD diamond material to have a colour which is not dominated by brown defects that do not contain nitrogen, but is instead dominated by the yellow hue due to the presence of single substitutional nitrogen. Likewise, there is a need for CVD diamond material where the electronic properties are dominated by single substitutional nitrogen, but not degraded by the other defects normally resulting from nitrogen in the growth process.